The present invention relates to processes for preparing calcium carbonate giving useful performances as a paper filler in the causticization step of the sulfate or soda pulp process, and more specifically processes for preparing calcium carbonate giving useful performances as a paper filler by defining the quick lime used and slaking and causticization conditions or other factors.
Normally, a filler is internally added to printing or writing papers for the purpose of improving brightness, opacity, smoothness, writing suitability, touch, printability or other properties. Such papers are made by the so-called acid process at about pH 4.5 or the so-called neutral process at a neutral to weakly alkaline pH of 7 to 8.5 using talc, clay, titanium oxide or the like as a filler. In the neutral process, domestic calcium carbonate can be used in place of expensive imported talc or clay. In recent years, neutral papers obtained by the neutral process have attracted interest because of the papers"" storability, and the number has been growing and will become increasingly widespread because of many advantages including paper quality, costs, environmental influences.
A feature of recent demands for paper is a significant growth in the field of leaflets, catalogs, pamphlets, direct mails or the like in commercial printing and in the field of books related to computer, multimedia and family computer to meet the development of the information age, magazines, collections of photographs, mooks, comics or the like in publishing. Thus, paper users increasingly desire to lower the cost and seek downgraded and lighter paper.
As the demands for inexpensive and light neutral paper increase as described above, the role of calcium carbonate as a filler will become very important. Calcium carbonate used as a filler in neutral papers includes heavy calcium carbonate obtained by mechanically dry- or wet-grinding a natural limestone and precipitated calcium carbonate chemically synthesized (synthetic calcium carbonate).
However, in heavy calcium carbonate obtained by grinding a natural limestone by a mill such as ball mill it is difficult to control the shape and it severely abrades plastic wires during paper making processes when it is used as an internal filler. Moreover, normal fine-quality papers or coated papers prepared with such a filler are insufficient in bulk, brightness, opacity, smoothness, writing suitability, touch, printability and other properties.
Recent advances in weight saving make the above problem graver. High-specific surface fillers (e.g. pulverized silica, white carbon), highly refractive fillers (e.g. titanium dioxide) or precipitated calcium carbonate (synthetic calcium carbonate) have been so far used as common means for improving the opacity of light printing papers.
The precipitated calcium carbonate is known to be prepared by (1) a reaction between carbon dioxide gas and a milk of lime obtained from a calciner of lime or the like, (2) a reaction between ammonium carbonate and calcium chloride in the ammonia soda process, (3) a reaction between a milk of lime and sodium carbonate in the causticization of sodium carbonate, etc. Among these processes, (2) and (3) have been less examined with respect to controlling the shape of the resulting calcium carbonate because both reactions produce it as a by-product and are now being replaced by novel formats for obtaining the main products. On the other hand, (1) has been widely studied with respect to techniques for preparing calcium carbonate in various shapes and have actually created some examples prepared on-site in paper factories, partly because the reaction system is relatively simple (water, slaked lime, carbon dioxide gas). However, the production costs of this process are very high because calcium carbonate is the sole product and so it can not satisfy users"" demands for cost-saving and can not be used, or at most in a greatly limited amount, for inexpensive types of papers.
A possible alternative is to use calcium carbonate by-produced during the causticization step for recovering and regenerating a digesting agent in a kraft pulp process, as a paper making material.
In the sulfate or soda pulp process, wood is digested with a mixed chemical solution of sodium hydroxide and sodium sulfide at high temperature under high pressure to isolate cellulose. Cellulose is separated as a solid phase and purified into pulp, while the chemical solution and other elements than cellulose eluted from the wood are recovered as a pulp waste liquor (black liquor) and concentrated and burned. During that time, the eluted elements from the wood are recovered as a heat source while inorganic matters based on sodium carbonate and sodium sulfide in the chemical solution are recovered and dissolved in water or a dilute chemical solution called weak liquor in which are dissolved a part of white liquor components generated when calcium carbonate sludge formed by the reaction shown below is washed to give a green liquor. This green liquor is mixed with a quick lime to produce calcium carbonate by the reactions (1) and (2):
CaO+H2Oxe2x86x92Ca(OH)2xe2x80x83xe2x80x83(1) 
Ca(OH)2+Na2CO3xe2x86x92CaCO3+2NaOHxe2x80x83xe2x80x83(2) 
This calcium carbonate can be prepared at very low cost because it is a by-product of the preparation of the main product white liquor. Moreover, it can be expected to improve the reactivity of the above reactions (1) and (2) and the clarity of the white liquor and to reduce waste, because the inside of the system can be clarified and the circulating lime can be highly purified by extracting calcium carbonate from the calcium circulating cycle (calcium carbonate, quick lime, slaked lime) in the causticization step which is conventionally a closed system.
However, it was difficult to control the shape of the thus obtained conventional calcium carbonate and it was always amorphous or massive including various shapes such as cube or hexahedron with large particle diameters similar to conventional heavy calcium carbonate and normal fine-quality paper or coated papers prepared with such a filler were insufficient in bulk, brightness, opacity, smoothness, writing suitability, sense of touch, printability or other properties. With recent large-scale paper-making machines producing paper at high speed, serious problems in plastic wire abrasion resistance and wet end yield also occurred.
Thus, it was difficult to efficiently and inexpensively prepare calcium carbonate, which is useful as a filler or pigment to give a good plastic wire abrasion resistance during paper making processes; it was difficult also to produce highly opaque fine-quality paper or coated paper while maintaining printing quality.
Considering the above situation, an object of the present invention is to provide inexpensive and light calcium carbonate with a controlled shape self-generated in the causticization step, which gives a good wire abrasion resistance during paper making processes and can be used to produce fine-quality paper or coated paper having high opacity and excellent printability or other properties.
As a result of careful studies to overcome the above problems, we found that they can be solved by slaking a quick lime containing calcium carbonate at a specific level or less with a white liquor to prepare a milk of lime and taking advantage of the causticization step of the sulfate or soda pulp process to continuously add a green liquor generated in the causticization step of the sulfate or soda pulp process in an amount comparable to that required for preparing a white liquor in conventional procedures to said milk of lime at a controlled loading rate and reaction temperature, and accomplished the present invention on the basis of this finding. According to the process of the present invention, the shape of calcium carbonate can be controlled without significant change in the conventional causticization step to prepare calcium carbonate in the form of spindle- or rice-like particles having a minimum diameter of 0.3 to 1.5 xcexcm and a maximum diameter of 0.5 to 7 xcexcm, which can improve brightness, opacity and wire abrasion resistance as a paper filler at a much lower cost than calcium carbonate obtained by the conventional reaction between a milk of lime and carbon dioxide gas. Additionally, the duration of the kiln operation can be shortened by extracting calcium carbonate from the causticization step or even be stopped depending on the amount of calcium carbonate extracted from the step, thus saving on the cost of the entire causticization step.
The quick lime used during the slaking reaction of the first step of the present invention may be the calcination product of a limestone based on calcium carbonate and/or calcium carbonate generated during conversion of sodium carbonate into sodium hydroxide in the causticization step of the sulfate or soda pulp process. The calciner used here may be any apparatus for converting calcium carbonate into a quick lime (calcium oxide) such as Beckenbach kiln, Meltz kiln, rotary kiln, Kunli kiln, KHD kiln, fluidized calciner, vertical mixing kiln.
Among impurities in the resulting calcium carbonate, coloring elements (Fe, Mn, etc.) must be especially controlled by suitably selecting a quick lime derived from a feed limestone containing less amounts of coloring elements to meet the purpose of the product paper. In the case of a quick lime recalcined in a rotary kiln or fluidized calciner or the like during the causticization step, a feed limestone containing less coloring elements may be supplied to the calcium circulating cycle of the causticization step or a controlled amount of the quick lime resulting from the calcination thereof may be used, depending on the conditions such as the ratio between calcium carbonate extracted outside the system and calcium carbonate recirculating in the system.
The calcium carbonate content in the quick lime is 0.1 to 10% by weight on the basis of the weight of the quick lime. If it exceeds 10% by weight, amorphous or massive calcium carbonate is produced, which has a low wire abrasion resistance and can not produce light coated paper with an intended quality. The content of 0.1% or less is uneconomic, because the energy required for calcination increases significantly or the calciner must be specially designed. The particle size of the quick lime is not specifically limited, but preferably 0.01 to 30 mm. If it is 0.01 mm or less, pulverization adds to the cost and dust occurs or transfer is troublesome. If it is 30 mm or more, homogeneous mixing can not be obtained by agitation during slaking.
When a white liquor is used for slaking the quick lime, it should be used at 80 to 160 g/L (expressed as Na2O in the same way as below), preferably 100 to 150 g/L in terms of total alkali or 30 g/L or less, preferably 25 g/L or in terms of Na2CO3. If the total alkali is less than 80 g/L, the concentration of the final white liquor is lowered and should be regulated before it is used for digestion. If the total alkali is more than 160 g/L, however, the resulting calcium carbonate has a low wire abrasion resistance without attaining an intended paper quality. If Na2CO3 is higher than 30 g/L, the resulting calcium carbonate also has a low wire abrasion resistance without attaining the desired paper quality. The lime level during slaking here should be 0.5 to 60% by weight, preferably 3.5 to 55% by weight on the basis of the quick lime before slaking. If it exceeds 60% by weight, the viscosity of the liquor becomes too high to practically agitate. If it is lower than 0.5% by weight, the productivity of calcium carbonate is unrealistically lowered.
When a green liquor is used for slaking the quick lime, it may be derived from the causticization step of the conventional sulfate or soda process and it should be used at 80 to 160 g/L (in which Na2CO3 represents 65 to 130 g/L), preferably 100 to 150 g/L (in which Na2CO3 represents 85 to 130 g/L) in terms of total alkali. If the total alkali is less than 80 g/L (in which Na2CO3 represents 65 g/L), the concentration of the final white liquor is lowered and should be regulated before it is used for digestion. If the total alkali is more than 160 g/L (in which Na2CO3 represents 130 g/L), however, the resulting calcium carbonate has a low wire abrasion resistance without attaining an intended paper quality. The lime level during slaking here should be 20 to 60% by weight, preferably 25 to 55% by weight on the basis of the quick lime before slaking. If it exceeds 60% by weight, the viscosity of the liquor becomes too high to practically agitate. If it is lower than 20% by weight, massive calcium carbonate particles are generated with low wire abrasion resistance, which can not attain the desired paper quality.
The quick lime and the white liquor or green liquor may be mixed using a means appropriately selected from conventional agitating blade- or pump-type extruders, kneaders and blenders depending on the viscosity of the liquor or slurry during mixing (see Handbook of Chemical Engineering published by Maruzen, Mar. 18, 1988).
The slaking temperature and period are closely related to each other. A short period suffices if the temperature of the aqueous solution used for slaking is high, while a long period is required if the temperature is low. The period is appropriately determined to meet the temperature condition of the quick lime used during slaking. As a standard, the reaction may be continued until the temperature rises due to heat generation during slaking stops. Practically, slaking is effective at a temperature as high as possible.
The green liquor in the causticization reaction of the second step of the present invention may be derived from the causticization step of the conventional sulfate or soda process and should be used at 80 to 160 g/L (in which Na2CO3 represents 60 to 130 g/L), preferably 100 to 150 g/L (in which Na2CO3 represents 85 to 130 g/L) in terms of total alkali. If the total alkali is less than 80 g/L (in which Na2CO3 represents 65 g/L), the concentration of the final white liquor is lowered and should be regulated before it is used for digestion. If the total alkali is more than 160 g/L (in which Na2CO3 represents 130 g/L), however, the resulting calcium carbonate has a low wire abrasion resistance without attaining an intended paper quality.
The green liquor is mixed with said milk of lime prepared in the first step at a loading rate of the green liquor to the milk of lime of 0.02 to 50 cc (green liquor)/min/g (quick lime), preferably 0.02 to 30 cc (green liquor)/min/g (quick lime). Loading rates lower than 0.02 cc (green liquor)/min/g (quick lime) are impractical because of low productivity, while loading rate higher than 50 cc (green liquor)/min/g (quick lime) are also impractical because a very high capacity pump is required.
Here, the milk of lime prepared from the quick lime in the first step may be replaced with a milk of lime prepared from calcium hydroxide at the same concentration as defined in the present invention.
The causticization reaction should be carried out at a reaction temperature of 20 to 105xc2x0 C., preferably 25 to 103xc2x0 C. Temperatures higher than 105xc2x0 C. are uneconomic because the boiling point under atmospheric pressure is exceeded to necessitate a pressure-type causticization system or the like. If the temperature is lower than 20xc2x0 C., however, amorphous or massive calcium carbonate is produced to thereby lower the wire abrasion resistance without attaining the desired paper quality. This is also uneconomic because the system must be designed for cooling, thereby adding to the cost.
Agitation during causticization reaction may be carried out using a means appropriately selected from conventional agitating blade- or pump-type extruders, kneaders and blenders which can homogeneously mix the milk of lime prepared in the first step and a green liquor (see Handbook of Chemical Engineering published by Maruzen, Mar. 18, 1988).
Under the conditions as mentioned above, calcium carbonate in the form of spindle- or rice-like particles having a minimum diameter of 0.3 to 1.5 xcexcm and a maximum diameter of 0.5 to 7 xcexcm can be prepared.
Calcium carbonate in various shapes obtained by the present invention gives better wire abrasion resistance as compared with calcium carbonate previously obtained in the causticization step and can be internally added to provide fine-quality paper and coated paper with excellent brightness, opacity, smoothness, writing suitability, sensation of touch, printability or other properties. From this it can be readily inferred that it can be used in newspapers, medium papers, printing papers, book papers, bill papers, dictionary papers, double-side ground wood kraft papers, bleached kraft papers, tissue papers, rice papers, Indian papers, paper boards, non-carbon papers, art papers, light coated papers, cast coated papers, wall papers, heat-sensitive papers or the like to provide them with firm body and excellent brightness, opacity, smoothness, writing suitability, sensation of touch, printability or other properties. It also can be used in various pigments to give excellent gloss, smoothness, printabilities or the like. In addition to papers, it can also be used in rubbers, plastics, paints, sealing agents, pressure-sensitive adhesives, fertilizers, etc.
Although the mechanism of the present invention has not been well explained, the calcium carbonate level in the quick lime and the total alkali level in the white or green liquor seem to have a significant influence on the properties of the milk of lime to influence the reaction state between the dissolved calcium hydroxide and carbonate ions during the subsequent addition of a green liquor. Sequential addition of a green liquor allows the dissolved calcium hydroxide to react with carbonate ions at a low ratio of carbonate ions during the initial stage, whereby crystals of calcium carbonate grow into a spindle or rice shape.
This calcium carbonate mainly has two features. Firstly, plastic wire abrasion resistance during high-speed paper making is improved. Secondly, opacity and brightness are improved when it is incorporated. The first feature results from the spindle or rice-like primary particles which are more liable to entangle with fibers to improve the yield, thus decreasing the amount of the filler passing through wire parts. The spindle or rice-like particles are also advantageous for improving abrasion resistance because they have a high aspect ratio and less sharp edges to lower the frictional resistance during contact with wires. The second feature can be explained from electron microscopic observations of the surface/section of paper, which show that spindle- or rice-like calcium carbonate fills gaps between pulp fibers as if they were microfibers and were stiff enough to form many minute air spaces to provide good opacity and brightness.